(556b) Nanofiber-Based Novel Electrode Architectures for Batteries and Supercapacitors

The ability to control the nano-architecture within electrode materials is critical to the development of high performing energy storage devices. We will present our work on development of nanofiber-based hierarchically structured materials for application in lithium-air batteries and electric double-layer capacitors. For supercapacitors, we report a facile method for obtaining extremely high surface area (>1500 m²/g) and uniformly porous carbon nanofibers.  As a first step, blends of polyacrylontritrle (PAN) and a sacrificial polymer in dimethyl formamide (DMF) were electrospun into non-woven nanofiber mats with diameters in the range of 200-300 nm.  Fast evaporation of solvent (~200 nl/s) and high elongational flow rate (~10⁵ s^-1) during electrospinning allowed us to prevent phase separation and develop a co-continuous morphology of PAN and the sacrificial polymer in the nanofibers. As a second step, electrospun nanofiber mats were subjected to stabilization and carbonization processes to obtain porous carbon nanofibers (CNFs) as PAN converted to carbon and the sacrificial polymer decomposed out to create intra-fiber pores. Unlike other studies so far, we chose the sacrificial polymer possessing a high decomposition temperature and chain rigidity which prevented fibers from shrinking and collapsing during the PAN stabilization process at ~280 ^oC. Here we demonstrated that using Nafion as a sacrificial polymer allowed us to obtain CNFs with specific surface area of up to 1600 m²/g without any activation process. We exhibit the tunability of the pore sizes within  CNFs by varing material composition. These materials exhibit a large specific capacitance (>200 F/g and >30 F/cm³) possibly due to the presence of a large fraction of meso-pores (2-4 nm) compared to activated carbons (pores <2 nm), which leads to an increase in the accesible carbon surface, thereby improving specific capacitance.      

For lithium-air batteries, we incorporate (via in-situ synthesis) manganese dioxide as the catalyst in porous carbon nanofibers to fabricate multi-functional air cathodes. The aim is to establish well-defined multi-phase boundaries within the cathodes such that all reactants; oxygen (via electrode pores), lithium ions (via electrolyte in pores) and electrons (via carbon) can simulatneously reach the catalyst nanoparticles for the oxygen reduction reaction to take place efficiently. Strutural characterization of all nanofiber samples was conducted  using scanning electron microscopy (for external nanofiber morphology), transmission electron microscopy of microtomed fiber sections (for internal structure),  and nitrogen sorption isotherms  (for pore size distribution, pore volume and surface area). Electrochemical performance was studied using cyclic voltammetry at different scan rates, galvanostatic charge-discharge measurements at different current densities, and electrochemical impedance spectroscopy (EIS).